This election cycle, msnbc.com is presenting the series Briefing Book: Issues '08 assessing issues and controversies that the next president cannot avoid once he takes the oath of office.

The American entanglement in Iraq has receded as a campaign issue in the past few months, partly because the number of American casualties has declined since last year. But Iraq will be an unavoidable dilemma for the next president.

Why it’s a problem
The United States has had troops in Iraq since March of 2003.

The current Defense Department outlays on Iraq, according to the Congressional Budget Office, total more than $9 billion a month, more than $108 billion a year.

More than 4,000 American military personnel have been killed and more than 30,000 wounded in Iraq since the 2003 invasion.

About 146,000 U.S. troops are now stationed in Iraq, making them unavailable for retraining or for deployment to other parts the world.

“The majority of military assets most effective in waging counterinsurgency and counterterrorist operations remain tied down in Iraq,” said Samuel Brannen, a fellow with the International Security Program at the Center for Strategic and International Studies (CSIS), a Washington think tank.

He added, “Most importantly it includes the so-called ‘high-demand, low-density assets’ that we cannot simply buy more of when we need them, such as Special Operations Forces, overhead reconnaissance capabilities (satellite, unmanned aerial vehicles, etc.), and intelligence analysts.”

This commitment of people and assets makes it more difficult for the United States to respond to threats in other parts of the world.

Where the candidates stand
In a speech on Iraq in July, Democratic presidential candidate Sen. Barack Obama said, “Iraq is not going to be a perfect place, and we don't have unlimited resources to try to make it one.”

Video: Petraeus on Iraq, U.S exit strategy
He said his goal was “a government that is taking responsibility for its future — a government that prevents sectarian conflict, and ensures that the al-Qaida threat which has been beaten back by our troops does not reemerge.”

He pledged to withdraw “our combat brigades at a pace that would remove them in 16 months.”

But he also said he would keep a “residual force” in Iraq to wipe out al-Qaida remnants, protect diplomats, and train Iraqi police and military forces.

Republican presidential candidate Sen. John McCain said recently that American forces “can only be responsibly withdrawn when it is clear that doing so will not jeopardize the tremendous gains for which our troops have fought.”

The right way to end the war in Iraq, he argued, is to withdraw U.S. troops “as Iraqi forces are able to assume greater responsibility, and as our enemies in Iraq are being increasingly weakened.”

Last year, McCain supported a major increase in the number of troops in Iraq, arguing that if the United States withdrew its forces the result would be “catastrophe in the form of increased Iranian influence. The Saudis are going to have to support the Sunnis. The Kurds are going to have increased problems with Turkey… and the bloodletting will increase, which means to me that we will be back in there, only under far more difficult circumstances, at some point.”

Unanswered questions
Obama has used the term “residual force” to describe the U.S. units he would keep stationed in Iraq if he were president.

In a speech on Iraq in 2006 he spoke of keeping in Iraq “a rapid reaction force to respond to emergencies and go after terrorists.”

But he has not explained how large this residual force would be. Nor has he said when its mission would be finished or when he would order it to be withdrawn.

McCain has not been willing to offer any schedule of troop withdrawal. He has said only that he favors “a further conditions-based withdrawal of U.S. forces” when that becomes possible.

He pointed out at a campaign event in New Hampshire last winter that the United States had kept soldiers in South Korea and Japan for more than 50 years.

A similar long-lasting deployment in Iraq, he said, would “be fine with me as long as Americans are not being injured, or harmed, or wounded, or killed."

Evolution and shifts in position
Obama opposed the Iraq war at the outset and often said during the Democratic primaries that "I have shown better judgment" than his rival Sen. Hillary Clinton (and, by implication, McCain) who "went along with George Bush on the war in Iraq."

Obama voted against the withdrawal of troops from Iraq (on June 22, 2006).

But he also voted (on May 24, 2007 and Nov. 16, 2007) for forcing the withdrawal of troops by cutting off funding for military operations in Iraq.

Obama said in March and April of 2007 that Congress had little choice but to approve the money to sustain the troops in Iraq.

“I think that nobody wants to play chicken with our troops on the ground,” he said.

“What you don't want to do is to play chicken with the president, and create a situation in which, potentially, you don't have body armor, you don't have reinforced Humvees, you don't have night-vision goggles,” he said.

In line with this reasoning, Obama voted for funding military operations in Iraq in 2006.

But in 2007 he decided to vote against funding. “This is a choice between validating the same failed policy in Iraq that has cost us so many lives and demanding a new one.” he said. “I am demanding a new one.”

Obama was an adamant opponent of President Bush’s “surge” of additional troops to Iraq in early 2007 and predicted it would fail.

"I am not persuaded that 20,000 additional troops in Iraq is going to solve the sectarian violence there. In fact, I think it will do the reverse,” he said in January of 2007.

But on Sept. 4 Obama said in an interview with Bill O’Reilly on Fox News, "I think that the surge has succeeded in ways that nobody anticipated. I've already said it's succeeded beyond our wildest dreams."

However, he said, “the Iraqis still haven't taken responsibility, and we still don't have the kind of political reconciliation” that is needed.

Early supporter
McCain was an early supporter of the invasion in 2003, although he said two weeks before it began, "I'm uncomfortable because you never know what's going to happen in a conflict."

Yet, he said then, it "would be a wonderful thing for the people of Iraq to be freed of a person (Saddam Hussein) who has killed a million of his own citizens, used weapons of mass destruction against them and has a brutal reign of terror...."

By June of 2003, McCain was criticizing the Bush administration for not begin candid with the American people about the cost of the Iraq operation.

He predicted the U.S. effort in Iraq would be "very long and difficult."

He said the American people "have the fortitude to support it because of the incredible benefits that can accrue from it, not only there in Iraq but throughout the region. But they need some straight talk about what the plan is, how long were going to be there and, sooner rather than later, how much the cost is going to be."

How they have votedMcCain voted for the 2002 resolution authorizing President Bush to use military force against Iraq.

Obama was not a senator in 2002, and thus didn’t vote on the resolution, but he spoke in his 2004 Senate campaign against the use of force.

In the 2004 campaign, Obama refused to criticize Democratic presidential nominee John Kerry for voting for the Iraq war resolution in 2002 and said he didn't know how he would have voted if he had been in the Senate at the point.

"I'm not privy to Senate intelligence reports," he said in 2004. "What would I have done? I don't know. What I know is that from my vantage point the case was not made.'"

McCain has voted to continue funding for military operations in Iraq. Obama has voted for funding, but switched to voting against funding in 2007.

Surprises for the new president
Brannen of CSIS said the surge succeeded in imposing some degree of peace because Iraq’s Sunni groups stopped fighting U.S. forces and went on the American payroll, freeing U.S. troops for counterinsurgency missions.

But now, “democracy in Iraq is grinding to a halt,” Brannen said.

“We’ve never had the long promised provincial and local elections and now we’re not going to have them this year. That’s a big problem.”

With the Iraqi government refusing to include the Sunni “Sons of Iraq” in the Iraqi security forces, Sunni frustration could result in new violence between Sunni and Shia forces, Brannen said.

“If the Sunnis don’t get to vote and don’t get to turn their new organization and cooperation into political power, things may quickly look like the kind of sectarian warfare that we’ve seen in the past.”

Another potential surprise that may await the new president: Muqtada al-Sadr’s nationalist movement might revive, with “people in the streets demanding that the United States leave immediately. That’s a shock for either Obama or McCain. Before they can get Iraq to the place you want it to be — stable, democratic, peaceful, not a safe haven for terrorists — the Iraqis are trying to throw you out.”

The U.S. forces might be able to withdraw to the Kurdish northern part of Iraq, but, said Brannen, “What happens if we do that, and then the Kurds declare themselves an independent state?”

The government of neighboring Turkey would object to this, as would other neighboring countries, Brannen said.